VARIATION AND HEREDITY IN TWINS 151 



ably find doubling in only one fetus. This final resort 

 to the early cleavages as the probable mechanism 

 responsible for the distribution of doubKng factors in 

 polyembryonic sets is taken advisedly after a thorough 

 canvass of all other possibiMties which have suggested 

 themselves. The idea is not incompatible with the 

 observations that have led to the budding hypotheses 

 of Patterson, as has already been shown, but fits in 

 better with the fission theory of polyembryony. 



Segregation of unit factors during cleavage may be 

 termed somatic segregation whether or not poly- 

 embryony be involved. Somatic segregation must, I 

 believe, take place in those forms that exhibit mosaic or 

 particulate inheritance; in a spotted animal, showing in 

 some areas the paternal and in others the maternal color, 

 •we certainly have a simple case of somatic segregation. 



It is in connection with bud variations or clonal varia- 

 tions, however, that we find a condition more nearly anal- 

 ogous to what appears to happen in armadillo embryos. 

 A green plant with colored flowers may pr6duce as a 

 bud variation a white-leaved branch with white flowers. 

 If such a white flower were self -fertilizing and produced 

 good seed from which white plants could be reared, we 

 should have more than mere somatic segregation, since 

 germinal segregation has gone hand in hand with somatic. 

 I have been informed that just such a situation is 

 sometimes realized for plants. 



Now in the armadillo the four fetuses are produced 

 agamically as fission products; they therefore constitute 

 a clone. There is every opportunity for clonal varia- 

 tion, and when a clonal individual gets a factor that 

 makes for doubling in the soma, it always has gametes 



